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Identifying Black-capped Gnatcatchers

Identifying Black-capped Gnatcatchers

Black-capped Gnatcatcher, California Gulch, AZ, 5/16/2009. Photo courtesy of John Schwarz, Birdspix.com (click for link). This photo was taken only three days after my recordings from the same location, so it may be of the same individual.

Just 30 years ago, the dapper Black-capped Gnatcatcher was ultra-rare north of the Mexican border.  Today it can be found with some regularity in decent numbers in several different locations in Arizona and New Mexico.  But separating it from the more numerous Blue-gray Gnatcatchers can be a real challenge, especially in winter, when the males don’t sport their namesake caps.

Voice is a key field mark, but good descriptions and recordings of Black-capped Gnatcatcher vocalizations have until recently been in short supply, and confusion about the vocal differences between eastern and western Blue-gray Gnatcatchers has compounded the issue.  Add Black-tailed Gnatcatcher to the mix, plus a dash of the genus-wide tendency to say unpredictable things, and you’ve got a recipe for confusion.

We’ll try to alleviate some of that confusion today.

Black-capped Gnatcatcher whines

The single best way to identify a Black-capped Gnatcatcher is by listening for one of its most common calls, a distinctive polyphonic overslurred whine that reminds some people of a kitten’s meow:

Male Black-capped Gnatcatcher, California Gulch, AZ, 5/13/2009.

This typical version of the call is strikingly similar to the distinctive mew of the California Gnatcatcher, but California is not found in the same regions as Black-capped.  Of course, Black-capped calls are also variable.  Here’s a rather odd version:

Black-capped Gnatcatcher call (variant with upslurred ending), Alamos, Sonora, Mexico, 7/3/2010.

And here’s a downslurred variant:

Black-capped Gnatcatcher call (downslurred variant), Alamos, Sonora, Mexico, 6/26/2010.

(Here’s another downslurred example for good measure.)

Beware the Blue-grays!

Not only is Blue-gray the gnatcatcher that looks most like Black-capped, it’s also the one that can sound most similar — especially the western population.  As we saw in the last post, the simple song of western Blue-grays is composed of overslurred whiny notes.  Usually the overslurred whines of Blue-grays are organized into short series during bouts of the “simple song,” while the similar notes of Black-capped are often (but not always) given singly.

When Black-cappeds give downslurred whines, they may be especially difficult to distinguish from the standard calls of western Blue-grays:

Western Blue-gray Gnatcatcher calls, Fremont County, CO, 5/15/2008.

 Black-capped Gnatcatcher rough calls

In addition to its trademark whines, Black-capped Gnatcatcher also gives some rough notes, possibly in alarm or as part of the simple song.   These rough calls could be mistaken for the sounds of a Black-tailed Gnatcatcher.

Male Black-capped Gnatcatcher calling in response to my whistled pygmy-owl imitation, California Gulch, AZ, 5/13/2009.
Male Black-capped Gnatcatcher, California Gulch, AZ, 5/13/2009.

Beware the Black-taileds!

Among the western gnatcatchers, the Black-taileds are usually considered the ones with the most distinctive voices — rough, harsh, noisy, hoarse, unmusical, and rather unlike the higher-pitched, polyphonic, whiny voices of their congeners.  But the rough calls of the Black-cappeds above encroach on traditional Black-tailed territory.  The last call above, in fact, is virtually identical to some calls of Black-taileds, like this example:

Ultimately, we still know very little about the voice of Black-capped Gnatcatchers.  They certainly sing a complex song like that of Blue-grays.  They probably sing something like the simple song of that species as well, but what comprises that simple song isn’t clear — this recording may be an example of it.  Rough notes appear to indicate agitation in at least some cases, but perhaps not always.

By far the best indicator of a Black-capped Gnatcatcher is the classic overslurred whine.  My experience indicates that this call can be heard from about 80% of Black-capped Gnatcatchers within five minutes of observation.  However, the species often gives variant calls for several minutes in a row, including downslurred or noisy versions that resemble those of the other two gnatcatcher species.

The take-home message?  Though their “classic” call is distinctive, Black-capped Gnatcatchers are more vocally variable than many people have given them credit for.  Identifying one in the field may require careful listening and a good deal of patience.

Eastern and Western Blue-gray Gnatcatchers

Eastern and Western Blue-gray Gnatcatchers

Eastern Blue-gray Gnatcatcher, Liberty, Missouri, 4/16/2011. Photo by Big Dipper 2 (CC 2.0)

The Blue-gray Gnatcatcher sounds different in the West than it does in the East.  As with geographic song differences in other birds, the differences in gnatcatcher songs might be of biological interest, perhaps encouraging the two groups not to mate with one another where their ranges meet.  However, the differences in song are not well understood by most birders, nor particularly well described in most field guides.  It doesn’t help matters at all that gnatcatchers are some of the most vocally complicated birds in North America.  The longer one listens to them, the more confused one might get.

The Blue-gray Gnatcatcher is one of the very few North American birds whose western population has actually been better studied than the eastern population, at least when it comes to vocalizations.  Most of what we know about the behavioral context of the different calls comes from a 1969 study by Richard Root that was conducted at the Hastings Natural History Reservation in Monterey County, California.  Root’s observations suggest (and my own field experiences corroborate) that western Blue-gray Gnatcatchers have two basic kinds of song: a louder, simpler one used primarily in territorial advertisement, and a quieter, more complex one used primarily in close-range courtship.  For today’s purposes, we’ll call them “simple song” and “complex song”.

Simple song

Here’s the simple song of western populations, which Root called the “advertising song”:

Western Blue-gray Gnatcatcher song, Fremont County, CO, 5/15/2008.

This is one of those magnificent spectrograms that deserves a moment of silent admiration.  The irregular spacing of the dark and light partials is not only visually striking, but a sure sign of polyphony, the simultaneous use of both sides of the bird’s syrinx, making for the distinctive whiny (some say “wiry”) tone quality of the gnatcatcher’s song.  This type of song is characterized by short series of 3-7 similar-but-not-identical notes, each one of which is typically overslurred.  A slight tendency toward up-and-down squiggling inside the individual notes on the spectrogram speaks to the slight burry quality of the sound.

This “simple song” comes in many variations across the western half of the Blue-gray Gnatcatcher’s range, even within the repertoire of a single bird, but the example above is quite typical.  Compare it with the simple song of eastern birds:

Eastern Blue-gray Gnatchatcher simple song, Bandera County, TX, 3/26/2008.

We still see the irregular stripey pattern that signals polyphony, but now only two dark partials dominate instead of three or four, and those two darkest partials are at a higher frequency and farther apart from one another than the partials in western songs.  This translates into a higher pitch with a thinner, less nasal tone quality.  And the tendency toward burriness is typically more pronounced, adding a grating quality to many notes that western birds most often lack.  Note shape also differs, with eastern birds showing  much less tendency toward the rollicking up-and-down patterns of western birds, but this mark is highly variable in both populations.

Complex song

Many people think of the complex song as the “true” song of gnatcatchers, probably because it better matches the traditional notion of a song as complicated and musical, but it is quieter and less frequent than simple song.  Complex song is given by males in close-range courtship of females as well as some territorial boundary conflicts with other males.  In both populations, the complex song is characterized by wildly diverse sounds, often including some mimicry, and herky-jerky rhythms that sometimes include a few repetitions of notes.  The end result can sound something like a Brown Thrasher song played back at higher speed.  But it’s usually easy to tell you’re listening to a gnatcatcher because of the liberal inclusion of individual whiny notes from the simple song.  These notes, in fact, are the best way to tell whether you’re listening to the complex song of an eastern or a western gnatcatcher.

Eastern Blue-gray Gnatcatcher complex song, Bandera County, TX, 3/26/2008.

Note that there’s a complete range of intermediates between simple and complex songs in both eastern and western birds — the elements appear to mix freely, and a significant percentage of songs may be difficult to put into one category or the other.

Calls

The word “call” gets used a lot to describe the simple song, but gnatcatchers do have non-song calls.  The calls are similar in quality to notes of the simple song, and they may integrade with it, so that it’s often difficult to tell calls and simple songs apart.  But here are a few examples of what I think are true calls:

Western Blue-gray Gnatcatcher call, Fremont County, CO, 5/15/2008.
Eastern Blue-gray Gnatcatcher calls, Scott County, MN, 6/13/2010.

As far as I can tell, the shape of the call note is pretty constant between populations and individuals: a nice even downslur.  The differences in pitch and tone quality of eastern and western birds exactly mirror the differences between the simple songs — eastern birds are higher-pitched and less nasal, and possibly less noisy as well.

Overall, the differences in voice between eastern and western Blue-gray Gnatcatchers are subtle, but consistent, and experienced field observers or those with recording equipment should be able to identify the two populations in the field by voice, even if they are out of range.  The breeding ranges of the two populations may meet or even overlap in west-central Texas or part of Oklahoma.  All the birds I recorded in the Texas hill country (Bandera and Kerr Counties) were clearly eastern birds, while the ones in Big Bend were clearly western, but I’m not clear on where the boundary is or whether intermediates occur along it.  I would love to get more information if anybody can share it!